1. Field of the Invention
This invention relates to a focusing position detecting device in an optical magnifying and observing apparatus which magnifies very small or fine patterns for observation with observing means such as a microscope.
2. Description of the Prior Art
LSI's of high integration density, bubble memories, photosensitive plates of image pickup tubes, and the like have very small or fine patterns of 2 to 3 .mu.m, and microscopes of high magnification are used for the inspection of the external appearance of these parts. The depth of the focus of such a high-magnification microscope is not more than 1 .mu.m, and automatic focusing with high accuracy is therefore demanded.
The basic principle of attaining focusing in an apparatus commonly used for optically magnifying and observing a fine pattern will be described with reference to FIG. 1, for a better understanding of the present invention.
Generally, an optical magnifying and observing apparatus comprises two optical systems, i.e., an observing system and a focusing position detecting system. However, in view of the fact that the present invention is specifically concerned with a focusing position detecting device, the latter or focusing position detecting system is only shown in FIG. 1, and the former or observing system is not shown to avoid confusion. Briefly describing, this observing system is to be understood to be a system in which visible radiation is projected through an objective lens 4 on the surface of an object 5 and radiation reflected therefrom is received for observation. (In such a system, a half-mirror of predetermined design is essentially required.)
In the example shown in FIG. 1, a laser beam 1 is employed for the purpose of focusing position detection. The laser beam 1 is diverged by a concave lens 2, reflected then by a half-mirror 3 and concentrated by an objective lens 4 on the surface of an object 5. In FIG. 1, the position of the object 5 is indicated by the solid line when the laser beam 1 is accurately focused by the objective lens 4, and a minute spot 6 is formed on the surface of the object 5 in that case. The beam 7 reflected from the surface of the object 5 passes through the objective lens 4 again and passes then through the half-mirror 3 to be concentrated on a point 8.
The combination of a pin-hole plate 9 formed with a pin hole 10 and a photoelective element 11 is provided for detecting the position of the concentrating point 8. The pin-hole plate 9 is arranged to oscillate in a direction of the arrow X (that is, in a vertical direction) in FIG. 1. FIG. 2 shows the waveform of the output from the photoelectric element 11. In FIG. 2, the horizontal axis X represents the direction of oscillation of the pin-hole plate 9, and the vertical axis V represents the level of the output from the photoelectric element 11.
When now the reflected beam 7 from the object 5 is concentrated on the point 8 in the pin-hole 10 of the pin-hole plate 9 as shown by the solid lines in FIG. 1, the output V from the photoelectric element 11 has a generally triangular waveform having a peak at the point 8 on the X-axis, as shown by a curve 12 in FIG. 2. The output V from the photoelectric element 11 has such a waveform since the pin-hole plate 9 is oscillating V in the direction of the X-axis around the concentrating point 8 of the reflected beam 7. It will be seen in FIG. 2 that the output V from the photoelectric element 11 becomes lower as the pin-hole plate 9 moves a greater distance away from the concentrating point 8.
Suppose that the surface of the object 5 shown by the solid line is displaced away from the objective lens 4 by a distance Z and is now located at a position 5' as shown by the broken line. In such a case, the reflected beam 7' concentrates on another point 8'. Therefore, when the pin-hole plate 9 is moved to the concentrating point 8', the output V from the photoelectric element 11 has a generally triangular waveform having its peak at the point 8' on the X-axis, as shown by another curve 12' in FIG. 2.
Thus, whether or not the surface of the object 5 lies on the focusing position of the objective lens 4 can be detected by finding whether or not the output V from the photoelectric element 11 at the point 8 is maximum, provided that the pin-hole plate 9 oscillates around the concentrating point 8 in FIG. 1 (the point 8 on the X-axis in FIG. 2). Therefore, the laser beam 1 can be focused by the objective lens 4 on the surface of the object 5 by moving either the object 5 or the objective lens 4 until the photoelectric element 11 generates its maximum output V.
FIG. 3 is a partly exploded perspective view of a prior art focusing position detecting device based on the principle above described. The structure and defects of the prior art device will now be described. In FIG. 3, the same reference numerals are used to designate the same parts appearing in FIG. 1. The observing system is disposed along the optical path indicated by the large arrow A, and its members are not especially shown to avoid confusion. A gas laser beam is used in FIG. 3, and the reference numeral 20 designates a gas laser beam emitter. The laser beam 1 emitted from the gas laser beam emitter 20 is converged by a convex lens 21 and is then diverged by a concave lens 2. After passing through a polarization beam splitter 22 and a quarter wavelength element 23, the laser beam 1 is reflected by a reflector 24 and is then concentrated by the condenser lens 4 to form a minute spot 6 on the surface of an object 5. The reflected beam 7 from the object 5 is concentrated by the objective lens 4 again, and, after passing through the reflector 24 and quarter wavelength element 23, is deflected by the polarization beam splitter 22 to be directed in a direction orthogonal with respect to the previous direction. Then, the reflected laser beam 7 passes through a concave lens 25 and is concentrated by a condenser lens 26 to form a laser spot. The pin-hole plate 9 having the pin-hole 10 is oscillated, and the beam passing throug the pin-hole 10 is detected by the photoelectric element 11.
The waveform of the output V from the photoelectric element 11 is shown on the right-hand side of the photoelectric element 11 in FIG. 3. It will be seen that the displacement x of the pin hole 10 of the oscillating pin-hole plate 9 is plotted on the horizontal axis 27 and the output V from the photoelectric element 11 is plotted on the vertical axis 28. The curve 12 represents the signal waveform of the output V from the photoelectric element 11. Therefore, the object 5 is moved in the vertical direction for the purpose of focus adjustment until the maximum output V appears from the photoelectric element 11 at the center of oscillation of the pin-hole 10.
However, a problem as pointed out presently is involved in the prior art device described above.
It is generally acknowledged that, in such an observing apparatus, it is necessary to suitably adjust the focus of the optical system for observation of an object, and it is also necessary to observe various portions of the object while, for example, moving the object in a lateral direction. The direction of movement of the object 5 while attaining automatic focusing is indicated by the arrow B in FIG. 3 and similarly by the arrow B in FIG. 1. With the movement of the object 5 in the direction of the arrow B, a pattern 5a on the object 5 moves naturally in the same direction. Since the factor of reflection varies depending on the portion of the pattern 5a, the intensity of reflected beam 7 varies also with the movement of the object 5.
Suppose now that a pattern 5a having a high reflection factor is present on the surface of the object 5, and such a pattern 5a is moving at a high speed in the direction of the arrow B in FIG. 1 and FIG. 3. The beam 7 reflected from the pattern 5a having the high reflection factor passes in front of the pin-hole 10 while the pin-hole plate 9 is under oscillation. Consequently, the output V from the photoelectric element 11 within the oscillation range X of the pin-hole plate 9 will have a wafeform as shown in FIG. 4, and, it will be seen in FIG. 4 that a peak output V.sub.5a appears when the reflected beam 7 from the pattern 5a having the high reflection factor passes in front of the pin-hole 10. In this case, the level of the output V.sub.5a is higher than that of the output V.sub.5 appearing at the exactly focused position 8. Consequently, when such a peak output V.sub.5a appears at the oscillating position x.sub.1 of the pin-hole plate 9, this position will be erroneously detected to be the exactly focused position of the object 5 relative to the objective lens 4.
Thus, the prior art device has been unable to accurately detect the true focusing position when a pattern having a high reflection factor is present on the surface of an object and has therefore been unable to attain the function of automatic focusing. While the above description has referred to the case in which a pattern having a high reflection factor is present on the surface of an object, the same applies also to the cases in which a pattern having a low reflection factor and a stepped pattern are present. In such cases too, the prior art device has been unable to attain the function of automatic focusing because a point at which the photoelectric element generates its maximum output does not necessarily coincide with the true focusing position. It has therefore been a common practice to detect the height of a pattern on the surface of an object by means of an air micrometer for the purpose of automatic focusing. However, blowing of a stream of air onto the surface of the object has resulted in blowing of fine dust particles at the same time. In the past, accumulation of fine dust particles on the surface of the object did not pose any substantial practical problem. However, with the recent trend of producing finer patterns in the fields of semiconductor and other industries, accumulation of such fine dust particles has frequently resulted in the production of defective patterns, and it has been the tendency that the air micrometer finds lesser applications in these industrial fields.